Input protection circuitry is an essential part of modern electronic circuits, particularly in the area of field effect transistor based devices. Extremely small delicate device structures are very sensitive to the high voltages which may be developed by an electrostatic discharge ("ESD") from a human body. When an electronic circuit is installed into a product, such an electrostatic discharge may destroy the part or reduce the quality or capacity of the part. These effects may be avoided by providing a means on the electronic circuit for dissipating electrostatic discharges. Complimentary metal oxide semiconductor ("CMOS") field effect transistor circuits are particularly subject to negative consequences of electrostatic discharges. In high density CMOS devices, an electrostatic discharge may develop an extremely high voltage which easily destroys the very thin gate oxides and very short channel devices of these circuits.
To protect CMOS devices against such high voltages, a silicon controlled rectifier ("SCR") structure may be used which is compatible with CMOS processing. Many SCR protection devices have very high threshold trigger voltages, frequently exceeding 50 volts. In many electronic circuits, a 50 volt threshold trigger voltage is insufficient to adequately protect certain devices on the electronic circuit. Some SCR protection devices having lower threshold trigger voltages nevertheless present alternative problems such as lower overall gain and lower maximum current.
Thus, a need has arisen for silicon controlled rectifier structures for electrostatic discharge protection, which are compatible with CMOS processing, which have low threshold trigger voltages, and which avoid presenting alternative problems such as lower overall gain and lower maximum current.